cmd/cgo: fix 'see gmp.go' to 'see doc.go'

Change-Id: I303edc9dfbf4185b5b461b121ab504f6ed9f8630
Reviewed-on: https://go-review.googlesource.com/c/go/+/330839
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/src/cmd/cgo/main.go b/src/cmd/cgo/main.go
index 03a662e..c6a0c52 100644
--- a/src/cmd/cgo/main.go
+++ b/src/cmd/cgo/main.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Cgo; see gmp.go for an overview.
+// Cgo; see doc.go for an overview.
 
 // TODO(rsc):
 //	Emit correct line number annotations.